"heroin addiction, where even 'safe' IV drug use is neither especially safe (you've got your overdoses and so forth) nor something worth doing (even if there was no such thing as AIDS or overdoses it would be better not to be a heroin addict)".
Doesn't that require some further explanation? If (and I have to undeline there that this is a thought experiment, so don't come back with well-reasoned cases about the actual evils of heroin) there were no physical disadvantages (and, me being a weenie, no injections)(and, to be sure, if there were no legal sanctions - but then, why would there be?) why would getting high on H be undesirable per se? It might be too expensive, or too time-consuming, or too distracting, but I seem to remember the same being true of sex. Aren't we accepting too readily the concept that enjoyment is inherently suspect? And the thought experiment does seem to be almost realised in the case of Ecstacy (the drug, not the orgasm), where the physical toll seems to be about level with that from the consumption of bacon.